Astronomical Firsts

How was the size of Earth first measured?

Earth's circumference was first accurately measured more than 2,000 years ago by the Greek astronomer Eratosthenes, who at the time lived in the Egyptian city of Alexandria. He had heard that in the nearby town of Syene midday sunlight shines straight down to the bottom of deep wells on the same day each year, indicating that the Sun was directly overhead in Syene. In Alexandria, however, sunlight on that date never reached the bottoms of wells, but instead fell upon the sides.

Eratosthenes reasoned that the difference in the angle of incoming sunlight was due to the curvature of Earth's surface, and so by measuring this angle, he related the distance between Alexandria and Syene to the total dimension of the globe.

On the day the Sun shone on the bottom of the wells in Syene, Eratosthenes measured the Sun's position in the sky over Alexandria. It was seven degrees away from the zenith, meaning Syene must be seven degrees away from Alexandria as measured on the circle that is Earth's circumference. Because seven degrees is about one 50th of a full circle (360 degrees), Eratosthenes simply multiplied the distance from Alexandria to Syene -- believed to have been about 515 miles (830 km) -- by 50. He calculated Earth's circumference at 26,000 miles (42,000 km), only five percent away from the modern accepted value of 24,901 miles (40,074 km).

Who built the first observatory?

Observatories, like so much of astronomy, are truly ancient features of human culture; retracing the steps of the first person to stand on a sacred hillside and search the sky for answers is truly an impossible task. Across the globe, however, such sites -- devoted to deciphering the heavens -- are found in considerable numbers. Unlike modern mountaintop facilities that probe the entire spectrum of light with computerized equipment, ancient observatories were placed in positions for optimal viewing with the eyes -- on hilltops or atop structures such as the ziggurats of ancient Babylon or the rooftops of Mexico's Monte Alban.

Ancient observatories often employed reference markers to the seasonal procession of the natural calendar; consider, for example, the familiar stone pillars of Stonehenge or the spokes of Native American medicine wheels. Such places were of considerable spiritual significance; astronomers, able to predict the motions of the Sun, Moon, and planets, were considered to be interpreting the will of the gods as written on the tableau of the skies -- a considerable, and occasionally dangerous, responsibility.

Who invented the telescope?

The true inventor of the first telescope is somewhat difficult to nail down. However, the first person to apply for a patent on a telescope was Hans Lippershey, a lensmaker in the Netherlands, in 1608.

The first astronomical use of the telescope is easier. After learning of the new device, the great Italian scientist Galileo Galilei designed and built his own. He turned his finest telescopes toward Jupiter, the Moon, and Venus in 1609 and 1610.

Who made the first map of the universe?

Maps of the stars are nearly as ancient as human culture itself; inscriptions describing the positions of stars and constellations appear on Egyptian temples, Babylonian tablets, and Chinese calendars produced thousands of years ago. By around 500 BC, Greek philosophers had begun to approach the mapping of the heavens in an almost modern fashion, making careful and repeated observations of the positions and motions of the stars and planets in an attempt to rationally explain the movements of the universe.

Credit for modern maps of the cosmos is often given to Harlow Shapley, who in 1917 published a map of the distribution of globular clusters and correctly concluded that our solar system was located a considerable distance from the center of our galaxy. Not long after, in 1924, Edwin Hubble was able to clearly resolve individual stars in other galaxies, and so was able to determine the distances to these "island universes" -- paving the way for future astronomers who would map the universe.